This invention relates to optical measurements, and particularly to the measurement of the chromatic dispersion of optical devices, particularly optical fibers used in telecommunication networks.
Chromatic dispersion is the difference in time delay that different frequencies (wavelengths) experience when being transmitted through an optical component or optical fiber. This phenomenon is caused by the frequency-dependence of the refractive index (n) of the material and the waveguide dispersion, which is related to the design of the optical fiber. Polarization mode dispersion (PMD) is the polarization-dependence of the time delay through an optical component or an optical fiber. Polarization mode dispersion and chromatic dispersion are related, in that they both reflect time delays through an optical fiber component. PMD is discussed first.
PMD is a distortion mechanism (like chromatic dispersion) that causes optical devices, such as single-mode fibers, optical switches and optical isolators, to distort transmitted light signals. The relative severity of PMD (which is a function of the wavelength of the transmitted light) has increased as techniques for dealing with chromatic dispersion have improved, transmission distances have increased, and bit rates have increased. Negative effects of PMD appear as random signal fading, increased composite second order distortion and increased error rates.
PMD is due to differential group delay caused by geometrical irregularities and other sources of birefringence in the transmission path of the optical device. For example, a single-mode fiber (SMF) is ideally a homogeneous medium supporting only one mode. In practice, it supports two propagation modes with orthogonal polarizations. When a lightwave source transmits a pulse into a SMF cable, the pulse energy is resolved onto the principal states of polarization of the fiber. The two groups of pulse energy propagate at different velocities and arrive at different times causing pulse broadening and signal distortion.
The PMD of a fiber is commonly characterized by two specific orthogonal states of polarization called the principal states of polarization (PSPs) and the differential group delay (DGD) between them. This can be described at an optical angular frequency, xcfx89, by the 3-component Stokes vector, {right arrow over (xcexa9)}=xcex94xcfx84{right arrow over (q)}, where {right arrow over (q)} is a unit Stokes vector pointing in the direction of the faster PSP, and the magnitude, xcex94xcfx84, is the DGD. Typical DGD values encountered in transmission systems range between 1 (picosecond) ps and 100 ps.
Known methods for determining PMD vectors include the Jones Matrix Eigenanalysis (JME) technique and the Mxc3xcller Matrix Method (MMM). Each of these techniques uses a tunable, continuous-wave laser and a polarimeter to measure the output polarization states for two (or three) different input polarization launches at two optical frequencies. The PMD vector is then calculated for the midpoint frequency. In addition to determining the output PMD vector, the Mxc3xcller Matrix Method determines the rotation matrix of the fiber at each frequency and thus the input PMD vector can be calculated.
Measurements of chromatic dispersion of optical components, spooled fiber, or installed fiber are important for predicting how severe the pulse distortion (and associated penalties) will be after transmission through the optical element(s). Chromatic dispersion is often measured with the modulation phase-shift method (B. Costa, et al., Journal of Quantum Electronics, Vol. 18, pp. 1509-1515, 1982). In this method, light from a tunable laser is modulated (usually with a sine wave at 1 to 3 MHz frequency) and launched into the optical element. The mean signal delay at the output of the optical element is measured using a network analyzer by referencing to the input. By measuring the delays for two frequencies, the chromatic dispersion at the average of the two frequencies can be obtained by dividing the change in delay by the change in frequency. The modulation phase-shift method is conventionally employed without control of the polarization of the signals launched into the optical element. For an element with no PMD, not controlling the input polarizations does not change the time delays measured or the calculated chromatic dispersion. But for birefringent elements or elements with PMD, the mean signal delays measured at the output will depend on the launched polarization of the signals. Therefore, the chromatic dispersion and the PMD both contribute to the time delay and cannot be easily separated.
The invention disclosed in this application details a method for accurately measuring the chromatic dispersion of an optical device in the presence of PMD. The chromatic dispersion measured is the intrinsic or polarization-averaged chromatic dispersion. Other known methods for measuring chromatic dispersion tend to give inaccurate results if the devices have PMD.
At the input of an optical device, typically an optical fiber link within an optical fiber telecommunications network, two different light signals, of the same optical frequency, but having different states of polarization that are orthogonal in Stokes space, are transmitted along the fiber and the mean signal delay of each of the light signals is measured. By repeating the mean signal delay measurement at multiple optical frequencies (i.e., at a different optical frequency for each set of time delay measurements), determination can be made of the first and higher-order intrinsic (polarization-averaged) chromatic dispersion of the device being measured. A method involving launching four test signals is also disclosed, where the launch polarizations are not required to be orthogonal.